1. Field of the Invention
The present invention relates to a sulfuric acid process for producing titanium dioxide, which discharges no industrial waste.
2. Related Background Art
In a production of titanium dioxide by a conventional sulfuric acid process, a titanium-iron containing substance like a titanium-iron ore (e.g., ilumenite and anatase) or titanium slag is reacted with sulfuric acid at an elevated temperature to produce a titanyl-sulfate-containing solid product composed mainly of titanyl sulfate, ferrous sulfate and ferric sulfate, and additionally of magnesium sulfate, manganese sulfate, chromium sulfate, vanadium sulfate, aluminum sulfate, sulfates of other elements, and unreacted substances. The sulfated solid product is dissolved in water, dilute sulfuric acid or a sulfate-salt-containing solution. In the dissolution, the molar ratio of sulfuric acid to titanium dioxide (hereinafter the ratio being referred to as an "acid value") have to be controlled to be not higher than 2.3 in order to precipitate the titanium ion in the solution by hydrolysis as insoluble titanium hydroxide. Accordingly, the concentration of the free sulfuric acid to promote dissolution of the solid reaction product cannot be raised unlimitedly. Further in the process, in order to reduce the amount of iron co-precipitated by hydrolysis, a reducing agent such as iron scrap is added to the solution to convert ferric sulfate therein to ferrous sulfate. After the reduction, crystalline ferrous sulfate (FeSO.sub.4.7H.sub.2 O) is deposited by cooling the solution to decrease the iron content in the solution and to produce a titanium-iron solution containing titanyl sulfate and variety of metal ions.
In conventional processes, the sulfate solution containing mainly titanyl-iron sulfate is added to a boiling dilute titanyl sulfate solution. Seeds for promoting the formation of titanium hydroxide and then boiling water are added to form insoluble titanium hydroxide. The resulting titanium hydroxide is separated from the liquid by filtration.
The separated titanium hydroxide, which contains adsorbed hydroxides of iron, manganese, vanadium, chromium, etc. contained in the raw material, is carefully rinsed repeatedly with a dilute sulfuric acid solution to remove other coexisting metals, and is subsequently rinsed by water to remove the adhered sulfuric acid. The titanium hydroxide is dehydrated and oxidized at an elevated temperature to give titanium dioxide.
This conventional process is disadvantageous in that the yield of the insoluble titanium hydroxide by hydrolysis is approximately from 94% to 95%, and the rest of the titanium is discarded as TiO.sub.2 at a concentration of from about 6 to about 9 g/l in the solution after the hydrolysis. The process is also disadvantageous in that minute amounts of scarce metals contained in the raw material such as titanium-iron ore and titanium slag are naturally not recovered and are discarded with the waste sulfuric acid after the hydrolysis. Even when the waste sulfuric acid is neutralized, the titanium dioxide and other metals in the waste sulfuric acid are similarly disadvantageously discarded. Further, titanium dioxide as well as other metals contained in the dilute sulfuric acid used for rinsing of the insoluble titanium hydroxide cannot be recovered efficiently and economically. Therefore such a dilute sulfuric acid solution is discarded without being recovered since evaporation concentration thereof is costly, which causes global oceanic pollution.
If the waste acid and the formed acid washing solution or water washing solution used for the titanium hydroxide is neutralized by use of calcium hydroxide which is an inexpensive alkali, it disadvantageously produces wet gypsum in an amount of 20 to 25 tons per ton of produced titanium dioxide, which costs a great deal in treating the large amount of the waste and creates difficulty in finding a site for discarding it. Accordingly, the sulfuric acid process for manufacturing titanium dioxide, which produces a large amount of waste, has been destined to decline in spite of its many advantages.
Improvements of the conventional methods for extracting titanyl sulfate from a sulfuric acid solution thereof are disclosed in U.S. Pat. Nos. 4,499,058 and 4,734,271.
In the method disclosed in U.S. Pat. No. 4,499,058, insoluble titanium hydroxide is once formed from titanyl-iron sulfate solution by a conventional hydrolysis; the resulting titanium hydroxide is separated by filtration; the filtrate after the hydrolysis is concentrated by evaporation to a concentration of total sulfuric acid concentration of than 500 g/l or more; and subsequently, most of the sulfuric acid and the titanyl sulfate in the aqueous solution is extracted by use of phosphorus type organic solvent. As a result, the titanium dioxide is recovered and the cost for treating the waste sulfuric acid is reduced. This method, however, has many disadvantages as listed below which prevents the commercialization of the method.
(1) The sulfuric acid having been extracted for the purpose of reducing the neutralization cost is stripped and transferred into a aqueous solution in the subsequent stripping stage, giving a high molar ratio of titanium dioxide to sulfuric acid (referred to as an "acid value" in this specification) in the recovered titanyl sulfate-containing solution, which prevents formation of insoluble titanium hydroxide by hydrolysis. PA1 (2) The need for a large amount of extracting agent to combine with the sulfuric acid to be extracted necessitates use of a large amount of organic solvent and a large amount of circulating organic solvent, which results in a heavy equipment investment. PA1 (3) The need for a high concentration of the phosphorus type extracting agent in the organic solution results in a high viscosity of the organic solvent, and the evaporation-concentrated waste sulfuric acid having a concentration of 500 g/l or more is in a state of viscous syrup, so that the extraction apparatus have to be operated at a heated state. PA1 (4) A large amount of water is used for regeneration of the organic solvent owing to a large amount of sulfuric acid extracted together with titanyl sulfate, recovering a dilute titanyl sulfate solution which cannot be utilized further in view of water balance. PA1 (5) Metals other than titanium, such as zinc, niobium, chromium, vanadium, aluminum, magnesium, manganese, and iron, cannot be economically recovered from the waste sulfuric acid solution after formation of insoluble titanium hydroxide, so that the amount of wasted matter cannot be reduced. PA1 (6) The dilute sulfuric acid-rinsing solution and the rinsing water obtained after rinsing of the insoluble titanium hydroxide formed by hydrolysis is not reusable, which prevents the reduction of neutralization cost and concentration cost. PA1 (i) The sulfuric acid concentration in the treated solution decreases as the extraction of titanyl sulfate proceeds, which will hinder the extraction of the titanyl sulfate. To compensate for this, the organic solvent has to be brought into contact a number of times with a 40-90% sulfuric acid solution, which renders the process complicated. PA1 (ii) The molar ratio of the titanium oxide to be extracted into the organic solvent to the sulfuric acid is controlled by an aqueous solution brought into contact in a stripping stage, which requires skill in process operation, as well as careful control of concentrations of titanium dioxide and sulfuric acid of the washing solution. PA1 (iii) With the decrease of the titanium concentration in the treated solution, the amount of extracted impurity metal ions increases. Therefore, the impurity metal ion concentration in the treating solution cannot be raised, in order to conduct extraction of the titanyl sulfate completely. PA1 (iv) With the decrease of the titanium concentration in the treated solution, the amount of co-extracted sulfuric acid will remarkably increase, and a large amount of washing solution is required for removing the co-extracted sulfuric acid. Therefore, the concentration of the impurity metal ions in the solution after the extraction cannot be raised, in order to conduct extraction of the titanyl sulfate completely. PA1 (v) With the progress of the extraction of titanyl sulfate, the sulfuric acid concentration in the treated solution decreases, which causes crystallization of ferrous sulfate (FeSO.sub.4.7H.sub.2 O) in the apparatus. Therefore, a crystallization apparatus for the ferrous sulfate has to be equipped in the extraction apparatus, which render the extraction apparatus complicated. PA1 (vi) The titanium ion remaining in the solution after the hydrolysis is not directly recoverable, which then requires the same concentration treatment as described in the U.S. Pat. No. 4,499,058. PA1 (vii) In conducting the titanium recovery operation described in above Item (vi), the contact of circulating organic solvent with sulfuric acid may cause contamination of titanium by a colored metal ion, such as, of chromium, vanadium, manganese, and iron. PA1 (viii) The method does not allow to recover economically all the metals other than titanium contained in the starting material, so that the discharge of noxious industrial waste cannot completely be prevented. PA1 the first step comprising bringing a sulfuric acid solution containing titanium ion together with one or more kinds of metal ions into contact with an organic solvent A composed of one or more extractants selected from the group consisting of oxygen-containing extractants and alkylamine extractants and a petroleum hydrocarbon diluent to extract the titanium ion as a sulfuric acid complex (titanyl sulfate) from the sulfuric acid solution; and bringing the organic solvent A containing the extracted titanium in a form of a sulfuric acid complex into contact with an aqueous liquid having been employed for rinsing insoluble titanium hydroxide formed in the second step and/or an aqueous liquid having been employed for stripping in the third step to strip the titanium in a form of sulfuric acid complex into the aqueous liquid and to regenerate the organic solvent A for recycling for extraction, PA1 the second step of treating the aqueous liquid containing the titanium in a form of sulfuric acid complex stripped from the organic solvent A for hydrolysis to form insoluble titanium hydroxide; and washing the titanium hydroxide with dilute sulfuric acid and or clear water, and PA1 the third step of bringing a part or whole of the raffinate of extraction in the first step into contact with an organic solvent B composed of an extractant selected from the group consisting of oxygen-containing extractants and alkylamine extractants and a petroleum hydrocarbon diluent to extract the titanium ion as a sulfuric acid complex from the raffinate; and bringing the organic solvent B containing extracted titanium in a form of sulfuric acid complex into contact with water, dilute sulfuric acid solution or a dilute titanyl sulfate solution to strip the titanium in a form of sulfuric acid complex to regenerate the organic solvent B for recycling to extraction.
In the method disclosed in the aforementioned U.S. Pat. No. 4,734,271, a sulfated starting material is dissolved in a sulfate salt-containing solution; iron scrap is added thereto to convert contained ferric sulfate to ferrous sulfate; the titanyl sulfate is extracted with a phosphorous type extracting agent from the sulfuric acid solution to separate other coexisting metal ions; subsequently iron is removed therefrom; and the titanyl sulfate is hydrolyzed. This method involves the same problems as the method of U.S. Pat. No. 4,499,058 in that the region of extraction of titanyl sulfate is large, and the amount of co-extracted sulfuric acid is large, and separately concentrated sulfuric acid is required additionally. In the former patent, the solution to be treated is firstly concentrated to raise the total sulfuric acid concentration and subsequently treated for extraction, while in the latter patent, separately concentrated sulfuric acid is brought into contact with the circulating organic solvent prior to and midway through the extraction of the titanyl sulfate, and the concentration is conducted with the solution after hydrolysis which is the same as in U.S. Pat. No. 4,499,058.
Moreover, the method disclosed in U.S. Pat. No. 4,734,271 is disadvantageous for the following reasons.